1. Field of the invention
The present invention relates to a circuit arrangement for bidirectional current limiting, in particular of ringing currents in conductor loops for telephone devices. The invention is advantageously applied in switching centre devices for telecommunication networks.
2. Description of the Prior Art
Ringing signals on two-wire lines indicate to connected subscriber devices a call or connection request and leads to the ringing of the dialled telephone or of the terminating subscriber device. In present-day modern telecommunication networks, the two-wire lines originally utilized for telephone traffic are used in parallel in high frequency ranges for digital data transport. High ringing currents on the corresponding two-wire line can then have a disturbing effect on the data traffic.
FIG. 5 shows a customary telecommunication network, used for telephone applications and also modern data transfer, at the switching centre end. In this case, provision is made of a telephone system POTS for voice communication on a two-wire line ZDL and a data system DAT for the broadband transmission of data via the two-wire line ZDL.
Both systems POTS, DAT are coupled to a splitter device SPL, which essentially feeds low-frequency signals SPR to the telephone system POTS and high-frequency data signals DT to the data system DAT. The splitter device SPL is generally embodied as a high-order filter comprising a plurality of inductors connected in series. A relatively large design is required inter alia because ringing signals which are transmitted via the two-wire line to the subscriber terminal equipment often cause large currents.
For ringing signal generation, a ringing signal generator RING is provided, which can be connected to the two-wire line ZDL via a switching device SE controlled by the telephone system POTS by means of control signals CTR. A ringing current then flows from the ringing signal generator RING via a first resistor R1′, the two-wire conductor loop ZDL or the splitter device SPL and via a second resistor R1 to earth GND. High ringing currents of above 100 mA may occur in particular when an off-hook identification is carried out. The inductors established in the splitter device SPL then rapidly achieve saturation and often cause data errors during the data communication that takes place in parallel via the two-wire line ZDL, the splitter SPL and the data system DAT.
In order to attenuate the disturbing influence of ringing currents, the prior art makes use of large designs of the inductors or of the splitter device SPL in order to avoid core saturations. Higher-order filters are generally expensive and complicated. This leads to increased costs and additionally requires matching of the splitter impedances to the voice impedances used in a respective country.